Conventional ceramic bodies, typically alumina ceramics, are manufactured by forming a raw batch of the desired ceramic ingredients in particulate form, (e.g. aluminum oxide), plus a binder, molding or otherwise forming the raw batch into a compacted body, and then firing the compacted body to sinter or vitrify the ceramic. During the firing operation, there is considerable shrinkage--generally from about 10% to 20%--which means that the compact must be made larger than the desired fired body. Also, because shrinkage is never exactly uniform throughout the body, there is always a certain amount of warpage or distortion. With good quality control, it is possible to maintain the warpage or distortion within the bounds of normal tolerances for many kinds of ceramic bodies; however, where extremely close tolerances must be met, shrinkage and any resulting warpage or distortion are serious problems.
Shrinkage occurs for one or a combination of two reasons. First, even if high pressure is used to mold or otherwise form the compact, the density of the compact is not as high as theoretically possible--there is always considerable porosity--to the end there is some shrinkage when the compact fires to a high density non-porous monolith. The inclusion of organic binder in the batch, which is desirable to add green strength to the compact, can contribute to the porosity of the compact in that the organic binder vaporizes or burns out early in the firing operation. Secondly, where the ceramic ingredients undergo chemical or crystalline transformation during the firing, if the ceramic formed is of greater density and lesser volume than the raw ceramic ingredients, then this also contributes to the shrinkage.
Various shrink-free ceramics have been proposed, as discussed in the article entitled "Porcelains Having Low-Firing Shrinkage", page 383, volume 43, No. 5 (1964) Ceramic Bulletin of the American Ceramic Society. U.S. Pat. No. 3,549,393 Elarde also discloses such ceramics. In the latter, the ceramic raw batch formulations include kyanite which converts to mullite and silica during firing with a resultant volume increase to compensate for the shrinkage which would otherwise occur. But the problem with such ceramics has been that they are relatively low in physical strength and wear resistance and hence are subject to deterioration when used in applications where high strength and wear resistance are important for optimum performance. One such application is that of dental crowns or the like dental appliances, and in this regard reference is here made to U.S. patent application Ser. No. 103,647 filed concurrently herewith, on Dec. 14, 1979, in the names of Ralph B. Sozio and Edwin J. Riley and entitled DENTAL APPLIANCE AND METHOD OF MANUFACTURE. Briefly that patent application is directed to a dental crown, or other dental appliance, and method wherein a cardinal feature is that the appliance is formed of shrink-free ceramic whereby a substantially perfect fit can be attained between the dental appliance and the prepared tooth to which the appliance is to be secured. The invention of this instant patent application provides an improved shrink-free ceramic which is not only shrink free but which is also possessed of increased strength and wear resistance, and hence of particular utility for the practice of the invention covered by said patent application in the names of Ralph B. Sozio and Edwin J. Riley.